The present invention relates to methods for reading out a matrix of elements in a Solid State Gamma Camera.
The use of solid state detectors for the detection of ionizing radiation is well known. Furthermore, the use of a mosaic of groups of detector electrodes on a single substrate of material such as CdZnTe has been mooted.
However, the application of such a matrix in a practical gamma camera is nearly obviated by the lack of a suitable fast readout system capable of reading out individual counts from the very large array of detector electrodes desirable for such a camera.
U.S. Pat. No. 4,672,207 describes a readout system for a mosaic of NXM scintillator/photodetector elements. In this system the photodetectors feed row and column amplifiers which indicate, for signals having the proper pulse height, that an event has occurred in the nth row and the mth column of the mosaic. However, this system requires a large number of scintillator crystals and, if applied to the solid state CdZnTe camera, as postulated above, would be unable to discriminate events which occur near or at the boundary between elements or to discriminate events which result in Compton scattering events.
In published PCT Application WO 95/33332 a method of reading out a matrix is described in which charge, generated as a result of events at points in the matrix, is stored at those points and the entire matrix is read out seriatim. This method, although mooted as being useful for a gamma camera utilizing CdZnTe, CdTe or a number of other materials at pages 45-48, is not capable of distinguishing individual events which would be necessary for the energy discrimination of events, used, for example, to eliminate events caused by Compton scattering.
It is an object of the present invention to provide a solid state gamma camera system having an improved readout system.
It is an object of some aspects of the present invention to provide a solid gamma camera system in which the outputs of individual pixels are recorded without the need to individually address the pixels.
It is an object of some aspects of the present invention to provide a solid state gamma camera in which events which occur near the boundaries of pixels and to some extent near the boundaries of crystals are properly detected.
It is an object of some aspects of the present invention to provide a solid state gamma camera in which the cells are all in a xe2x80x9ctalk-onlyxe2x80x9d mode, in which no noise producing interrogating signals are necessary and in which each pixel transmits its data immediately after it detects an event.
It is an object of some aspects of the invention to provide a system which detects events which produce signals in more than one pixel, without collision of the data which is generated on these adjoining pixels.
A solid state gamma camera, in accordance with a preferred embodiment of the invention, is made up of a mosaic of crystals of CdTe (or alternatively of CdZeTe, HgI2, InSb, Ge, GaAs, Si, PbCs, PbS or GaAlAs). One side of each crystal is preferably covered by a single, common, electrode and the other side of the crystal is preferably covered by a rectangular (preferably square) matrix of closely spaced electrodes. This matrix of electrodes defines the cells or pixels of the gamma camera image. In a preferred embodiment of the invention, the matrix comprises 16xc3x9716 elements having a size of 2xc3x972 mm. However, the size of the elements and the matrix size may vary over a relatively wide range depending on the desired spatial resolution and count rate. In particular, crystal sizes of 1xc3x971 to 4xc3x974 mm appear to be reasonable in the practice of the invention.
Generally, a rectangular mosaic of crystals each with its associated matrix of elements is used to provide a camera of the required size. This mosaic may have a dimension of 20xc3x9720 crystals or greater.
When a gamma ray impinges on the crystal, energy which is transferred to the crystal creates charge carriers within the normally insulating crystal such that it becomes temporarily conducting. When a high voltage is applied between the electrodes in the matrix and the common electrode, this charge generation results in current flow between them. This current generally lasts between 50 and 600 nanoseconds, depending on the depth of penetration of the gamma ray prior to its interaction with the crystal, and the crystal quality. The total charge collected by the matrix of elements is substantially proportional to the energy of the absorbed gamma ray. In this regard, each element can be considered as a signal source which produces a signal when a gamma ray absorption event occurs at or sufficiently close to its associated pixel.
In principle, the current resulting from a particular event (i.e., an absorbed gamma ray) should be limited to a single element of the matrix. However, a number mechanisms act to cause current to be measured at, generally, adjoining matrix elements.
One type of mechanism which induces current in more than one electrode is when an event occurs at or near a boundary between two or four matrix elements. Clearly, an event which occurs precisely at the boundary will cause an equal division of current between the adjacent two or four electrodes. Furthermore, events which occur near a boundary will also cause current to flow in adjoining elements since the gamma ray creates a small, but finite cloud of charge carriers which may overlap more than one cell and which diffuses and widens during its travel toward the electrodes. Thus, part of the current associated with an event near the boundary will be detected in an adjacent pixel element.
For each of the above effects, the energy of the gamma ray is deposited at substantially one point in the crystal and its effects are measured at more than one pixel element. Some events do not deposit their energy at only one point in the crystal. Rather they may undergo Compton scattering so that a portion of their energy is deposited at various points in the crystal. Each of these energy deposits causes currents to flow in corresponding pixel elements.
The above effects are dependent on both the energy of the gamma ray photons and the depth of penetration of the photon when it interacts with the crystal. Higher energy photons produce a larger electron cloud and have a higher probability of Compton scattering, such that, for 500 KeV photons, less than half will deposit their energy at a single point. The depth of penetration of the photon will determine the amount of spreading of the electron cloud prior to its being collected by one or more of the matrix elements.
While there is a relatively large probability that current will be collected in neighboring electrodes, the probability that current will be collected by non-neigboring electrodes is small, for the energies used in Nuclear Medicine.
The determination of the position and energy of an event, especially for the situation where more than one matrix element receives current from the event, requires that (i) current generated by each event be separately received for each event and (ii) that the response at each matrix element be separately received, or at least that all currents for a particular event be added to give a proper measure of the energy of the event. This would appear to require that each pixel be connected, separately or in a multiplex fashion, to the main data processing computer. Such a connection would be impractical.
In accordance with a preferred embodiment of the present invention a pre-processing and multiplexing unit is attached to each crystal. This unit, referred to herein as an xe2x80x9cASICxe2x80x9d unit, determines the distribution of charge (i.e., energy) associated with each event and the position of the event. For events whose charge is associated with more than one pixel, the ASIC unit determines the amount of charge associated with each of the pixels. It is this reduced amount of information, namely, the energy associated with each pixel which is involved in an event and the position of each of these pixels which is collected.
In accordance with a preferred embodiment of the invention, the pixels on each crystal are grouped in K identical rectangular groups of nxm pixels, designated pi, (i=1,2, . . . K) in a raster manner. The positions of the pixels in each group are designated as Pj (j=1,2,L=nxm) in a raster manner. Thus, Pij completely define the pixel in the crystal. The preconditioned voltages from electrodes having the same value of i are connected to the inputs of the same ASIC. Under normal circumstances, in which each element is separately interrogated, Kxnxm lines would be needed.
The basis for a reduction in the number of lines required to specify the position and strength of an event in the crystal is based on the fact that most events produce charge and current in one pixel and at most in 2-4 contiguous pixels. Thus if nxm is at least 2xc3x972, signals can only be generated in no more than one pixel for each of the K groups. The pixels may be in adjacent groups, however, the i designation of the pixel in the adjacent groups will be different for any event.
Each ASIC produces a coded output of the position of the group from which the signal was received, a voltage proportional to the charge generated at the electrode and, preferably, an output which indicates that an event has occurred.
For example, consider a crystal having a matrix of 16xc3x9716 pixels grouped into 64 (8xc3x978) groups of 2xc3x972 pixels. Such a crystal has four ASICs, one for each position in the group. Each ASIC (having 64 input lines, one for each group) thus requires 8 output lines to completely describe the portion of the charge generated at the electrodes. One of the lines carries the signal amplitude (analog) and six lines are required for the address. In addition a eighth line preferably carries the xe2x80x9cevent occurredxe2x80x9d signal.
Associated with each crystal is a module carrier which carries the ASICs associated with the crystal, e.g., four ASICs for the preferred embodiment. The total number of lines need to specify the position and intensity of an event in a crystal is thus, for the preferred embodiment, 8xc3x974=32 lines. While the number of xe2x80x9cevent occurredxe2x80x9d lines could be reduced by combining the signals from the various ASICs, it is preferable to utilize a separate xe2x80x9cevent occurredxe2x80x9d line for each ASIC to avoid residual signals on the other lines being considered by the computer.
It is understood that the time required to detect an event internally inside the ASIC depends on the time required to collect all the charge (a few hundred nanoseconds to 1 microsecond or more depending on the circuitry used). However, the time the lines are busy may be much shorter, since this time can be as short as the time it takes to stabilize the analog signal on the output lines plus the time it takes for the A/D conversion at the computer end. Using presently available components a xe2x80x9cline busyxe2x80x9d time of 100 nanoseconds or even 50 nanoseconds is easily attainable. This xe2x80x9cline busyxe2x80x9d time is the factor which limits the rate of event collection. At the end of this time the ASIC is preferably reset.
Generally, a gamma camera will comprise a number of crystals in a mosaic. If the speed required of the camera is slow, i.e., it is sufficient to detect one event per event time cycle, a further reduction in the number of lines from the camera into the computer can be achieved. In this case the energy outputs from all the ASICs are summed and the addresses are combined to give the address of the events in a larger space. For additional crystals, additional address lines will be required. Thus, if a mosaic of 16xc3x9716 crystals is utilized, an additional 8 lines will be required, bringing the total number of lines for the preferred embodiment to (8+8)*4=64. These lines are grouped into four identical buses of 16 lines each. However, this reduction in lines may result in collisions at rather low event rates.
The count rate of the system can be improved substantially by further grouping of the crystals. For example, if the crystals are grouped in groups of four (2xc3x972), and the crystals having the same position are grouped together, the system will require a total of [(6+8)*4]*4=224 lines.
Further count rate improvement can be obtained by increasing the size of the groups, thereby increasing the number of lines required.
It is thus seen that the present invention allows for a trade-off between the number of lines and the speed. In general, 32 lines is sufficient for most systems.
It should be understood, that were the electrodes connected directly to the computer, the number of lines required for a system having a mosaic of 16xc3x9716 crystals, each having 16xc3x9716 pixels would be 65536, a completely unwieldy number. Even the use of multiplexing and fast sampling would still require a very large number of lines.
The two most demanding applications for gamma camera are first pass and coincidence modes. In first pass a radio-isotope is injected into a vein leading to the heart. The first pass of the nearly undiluted radioactive material through the heart is measured to assess the heart function. Since the measurement time is very short, high count rates must be achieved in order to collect meaningful statistics. Rates of 400,000 counts per second or more may be encountered during first pass. Since the projection of the heart is approximately 100 cm2 the rate density is about 4,000 counts/cm2-sec. On the assumption that half the events (on the average) split into two adjacent cells, the rate of threshold crossing is one and one half times the event rate or 600,000 counts per second (cps) for the system and 600 cps/cm2.
On the individual cell level, where the size is very small, even assuming a band pass filter with a time-constant of 1 or several microseconds, there is no practical limitation on the system rate.
On the ASIC level, the ASIC resets its channels once the data is transmitted from one of its cells. If an event is detected in one cell after another cell has crossed the threshold, but before the other cell transmits its information and resets the ASIC, that information will be lost. This time is set by the one-shots of FIG. 10A at 420 nanoseconds, which leads to a nominal rate of 2.4xc3x97105 cps/ASIC. Since each ASIC serves 64 cells, the nominal density is 9.4xc3x97104 cps/cm2, which poses no problem in achieving the required count rate.
On the system level, there are four buses, each is busy for 100 nanoseconds while data is transmitted. This leads to a maximum rate of 106 cps/buss or a system rate of 4xc3x97106 cps versus the 6xc3x97104 cps required. This would result in an acceptable loss of counts. Alternatively, the busy time of the busses can be reduced by at least a factor of two by using faster A/D convertors.
Operation in a coincidence mode requires rates of up to 106 per head. Since this is close to the limit for the preferred embodiment, for such systems a smaller grouping with a larger number of lines may be preferred.
The spatial response of a detector head comprised of a multitude of discrete detector cells is space variant. A small object placed above the cell center will produce an image significantly different from one placed at the boundary of two cells. A space invariant response can be achieved by moving the detector cells with a controlled motion parallel to the detector plane, such that the object is viewed, preferably with equal probability by all points in an area at least equal to the cell size. If this motion is monitored and compensated for, preferably on the fly, on an event by event basis, two performance improvements may result:
a) the detector performance will be spatially invariant with a resolution (separation power) of one cell.
b) the accuracy of location measurement will be equal to that of the accuracy of the determination of the motion of the head.
The dithering scan length should extend over at least one cell, preferably over an integer number of cells, for example one or two cells or more.
Data which is acquired at the varying positions of the head is reframed into an image pixels which correspond to fixed positions with respect to the patient. The size of the image pixels is smaller than, and generally much smaller than, that of the detector cells.
There is therefore provided, in accordance with a preferred embodiment of the invention, a gamma camera head comprising:
a plurality of signal sources, each associated with a pixel position, each said source producing a signal when a gamma ray absorption event occurs at or sufficiently close to its associated pixel, wherein said plurality of signal sources is associated with a contiguous extent of pixels; and
a plurality of electronic circuits, each of which receives signals from at least two of the plurality of signal sources, wherein each said circuit receives said signals only from sources associated with con-contiguous pixels.
Preferably, at least two of the sources are connected by a common connection, preferably a permanent common connection to each of said plurality of sources.
There is further provided, in accordance with a preferred embodiment of the invention, a gamma camera head comprising:
a plurality of signal sources, preferably solid state sources, each associated with a pixel position, each of said sources producing a signal when a gamma ray absorption event occurs at or sufficiently close to its associated pixel;
an electronic circuit which receives non-multiplexed signals from all of said sources; and
a plurality of signal lines connecting all of said sources to said circuit, wherein at least one of said lines connects more than one source to said circuit.
Preferably, the circuit comprises a plurality of circuits, each of which is connected by a common connection, preferably a permanent common connection, to at least two of said plurality of signal sources.
Preferably, signal source is connected to only one of said plurality of circuits.
In a preferred embodiment of the invention, each electronic circuit produces a signal related to an energy of the event whenever any of the signal sources from which it receives signals produces a signal greater than a predetermined threshold.
Preferably, said pixels are grouped into contiguous groups of contiguous pixels and wherein each of said plurality of circuits receives signals from only one pixel in each group.
Preferably none of said plurality of circuits receives said signals from contiguous pixels in two adjoining groups.
In a preferred embodiment of the invention, the number of said common connections is less than or equal to the number of contiguous pixels in a group. Preferably, the pixels are grouped in contiguous groups of contiguous pixels and wherein each of said plurality of circuits receives signals from only one pixel in each group.
There is further provided, in accordance with a preferred embodiment of the invention, a gamma camera head comprising:
a matrix of signal sources, preferably solid state signal sources, each associated with a pixel position and grouped into a plurality of geometrically similar groups, each group having a plurality of contiguous pixel elements; and
a plurality of electronic circuits, each of which receives signals from one pixel element within each of a plurality of groups, each said pixel element having a similar geometric position within its respective group.
Preferably, each signal source produces a signal when a gamma ray absorption event occurs at or sufficiently close to its associated pixel position.
In a preferred embodiment of the invention, each electronic circuit also produces at least one signal indicating in which group of pixels the signal was generated.
Preferably, each electronic circuit also produces at least one signal indicating that an event has occurred, the indicating signal preceding the energy signal in time.
In one preferred embodiment of the invention each group comprises four pixel elements. In other preferred embodiments of the invention each group comprises 2 or 9 pixel elements.
Preferably, the sources transmit said signals to said circuit independent of any interrogating signal to the sources.
Preferably, the sources are each associated with an array of contiguous areas on the camera, such that said signals represent events which occur at or near the associated area and wherein said circuit identifies events which generate signals in sources associated with two neighboring areas.
In a preferred embodiment of the invention the signal sources are associated with at least one normally insulating crystal in which free charge is produced when a gamma ray is absorbed therein. In a preferred embodiment of the invention the signal sources comprise a matrix of conductive elements on the crystal which collect the free charge.
In a preferred embodiment of the invention the at least one crystal comprises a mosaic of such crystals.
There is further provided, in accordance with a preferred embodiment of the invention, a gamma camera for imaging radiation emitted from or transmitted by an object, comprising:
a gamma camera head having a front, input, surface, and which produces signals when a photon associated with the radiation is detected by the head, indicative of the position of the detection on the input surface, at a given resolution; and
a dithering system which differentially translates the detector head or the object in at least one direction parallel to the input surface by an amount at least equal to the given resolution but less than 50 times the given resolution during acquisition of the events.
There is further provided, in a preferred embodiment of the invention, a gamma camera for imaging radiation emitted from or transmitted by an object, comprising:
a gamma camera head having a front, input, surface, and which produces signals when a photon associated with the radiation is detected by the head, indicative of the position of the detection on the surface, at a given resolution; and
a dithering system which differentially translates the gamma camera head or the object in two directions parallel to the front surface by an amount at least as large as the given resolution during acquisition of the signals.
Preferably the amount of differential translation is greater than twice or four times the given resolution.
In a preferred embodiment of the invention, the gamma camera includes circuitry which receives the signals and an indication of the position of the head and which distributes the events into an image matrix of pixels having a matrix resolution finer than the given resolution, said image matrix being referenced to the object.
Preferably, the event is distributed into an image pixel having a reference point closest to a reference point in the head, translated by the position indication.
In a preferred embodiment of the invention, events acquired at a plurality of head positions having a distance therebetween smaller than the given resolution are distributed to said image matrix.
In a preferred embodiment of the invention, the gamma camera includes an imaging system which provides an image of the distribution of the detected radiation based on the signals, the image having a second resolution which is substantially constant over the surface.
There is further provided, a gamma camera for imaging radiation emitted from or transmitted by an object, comprising:
a gamma camera head having a front, input, surface, and which produces signals when a photon associated with the radiation is detected by the head, indicative of the position of the event on the surface at a given resolution; and
an imaging system which provides an image of the distribution of the detected radiation based on the signals, the image having a second resolution which is substantially constant over the surface.
Preferably the second resolution is substantially equal to the given resolution. The matrix resolution is preferably finer than the given resolution by any factor, for example by a factor of at least two or four.
In a preferred embodiment of the invention, radiation sources, whose captured radiation is spaced by a distance greater than the sum of the given resolution and the image pixel, will be separately imaged as sources which have a center spaced by the distance, substantially independent of the position of the capture of the radiation on the surface. Preferably, the image of a line source of constant width will have a constant width along its length for any inclination of the line on the surface. Preferably, the image of two point sources will have a substantially constant spacing independent of their position on the surface.
In a preferred embodiment of the invention, the gamma camera head comprises an array of detector elements, preferably solid state detectors, which produce said signals in response to the detection of the photons and wherein the spacing of the elements is substantially equal to the given resolution.
In a preferred embodiment of the invention the gamma camera head incorporates an array of solid state detector elements which produce said signals in response to the detection of the photons.
There is further provided, in accordance with a preferred embodiment of the invention, a gamma camera head for imaging gamma rays emitted from or transmitted by an object comprising:
a plurality of detectors, each having a physical extent and spacings which define a physical resolution of the head, each detector producing a signal when the head detects a gamma ray which is associated with a cell in an acquisition matrix having said physical resolution; and
an image matrix into which said events are individually distributed, wherein said image matrix has a resolution which is finer than the physical resolution.
Preferably, the image matrix is referenced to the object and the distribution into the finer image matrix is determined by the amount of the translation.
Preferably, the events are subsequently redistributed into a second image matrix having a resolution different from the image matrix or physical resolution.
In one preferred embodiment of the invention, the second image matrix has a resolution which is poorer than the physical resolution by a non-integral value.
There is further provided, in accordance with a preferred embodiment of the invention, a gamma camera head for imaging gamma rays emitted by or produced in an object comprising:
a plurality of detectors, preferably, solid state detectors, each having a physical extent and having a spacing therebetween which define a physical resolution of the head, each detector producing a signal when the head captures a gamma ray which is associated with a pixel in an acquisition matrix having said physical resolution; and
an image matrix into which said events are distributed, wherein said image matrix has a resolution which is poorer than the physical resolution by a non-integral value.
There is further provided, in accordance with a preferred embodiment of the invention, a gamma camera comprising:
a gamma camera head as described above; and
an imaging system which provides an image of the gamma rays based on the signals, having a resolution which is substantially constant over the surface of the head.
The invention will be more clearly understood from the following description of preferred embodiments thereof in conjunction with the drawings in which: